Medical professionals and technicians often review biological specimen slides in order to analyze whether a patient has or may have a particular medical condition or disease. For example, a cytological specimen on a slide may be prepared and examined to determine the presence of malignant or pre-malignant cells as part of a Papanicolaou (Pap) smear test, or other cancer detection tests. For this purpose, collected cells can be stored in a liquid preservative, and a slide having cells of the specimen can be prepared using a filter transfer technique, as described in U.S. Pat. Nos. 6,572,824, 6,318,190, 5,772,818, 5,364,597 and 5,143,627 and U.S. Publication Nos. 2008/0145887 and 2007/0099291, the contents of which are expressly incorporated herein by reference as though set forth in full.
For example, one known automated slide preparation system includes a container or vial that holds a cytological specimen having tissue or cells in solution. An end of a filter or membrane is inserted into the fluid, and short pulses of vacuum are applied to draw short “sips” of fluid into the filter cartridge. The vacuum pressure decays as cell containing fluid is drawn across the membrane and into the filter cartridge to collect cells on the filter. The “decay” of this temporary pressure drop, specifically the change in pressure inside of the filter over time, is used to calculate the amount of cells collected on the filter or membrane or the “membrane occlusion percentage”. The vacuum level decays faster when the membrane has no cells or only a few cells compared to when the membrane has collected a larger number of cells. Other systems utilize a mass air flow sensor to detect filter coverage rather than vacuum decay. This is done by detecting the rate of air flow through the filter and through the air flow sensor. The air flow rate changes as cells are collected on the filter. The air flow rate is higher when no cells or only a few cells are on the filter and decreases as the filter becomes increasingly clogged with cells.
While current systems and methods for applying cells to a specimen slide using filter transfer techniques have been effectively utilized to prepare specimen samples on a slide, they have a number of shortcomings in that they do not account for different cellular densities of specimens. Further, attempts to increase processing throughput with certain known systems may lead to application of excessive vacuum to certain specimens and increased cell clustering.